[foreign-memaccess+abi] RFR: Investigate alternate strategy to acquire resource scopes [v3]

[Paul Sandoz]

On Wed, 21 Apr 2021 14:04:02 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> The code in this PR demonstrates a possible, alternate approach to resource scope acquire.
>> 
>> Instead of having `ResourceScope.Handle` be `AutoCloseable` (which is problematic, given that `javac` issues warning when an auto closeable is not used inside the body of the try block) this patch adds a new method on resource scopes, namely `ResourceScope::release(Handle)`.
>> 
>> The semantics of this method is similar to the previous `Handle::close` - one thing that is different is how implicit scopes are handled - instead of trying to keep the implicit scope alive through the handle instance, here we simply return a "dummy" handle, which does nothing.
>> 
>> The idea is that if you write idiomatic code like:
>> 
>> 
>> var handle = scope.acquire();
>> try {
>>    // critical region
>> } finally {
>>    scope.release(handle); 
>> } 
>> 
>> 
>> This will work for both explicit scopes (since acquiring actively prevents calls to `ResourceScope::close`) - but also for implicit scopes, since the `release` call inside the finally block will effectively require the scope to be reachable at this stage (see reachability fence inside `ResourceScopeImpl::release`).
>
> Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - Merge branch 'foreign-memaccess+abi' into acquire_release_scope
>  - Add reachability fence on ResourceScope::release
>  - Initial push

src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/ResourceScopeImpl.java line 131:

> 129:         try {
> 130:             Objects.requireNonNull(handle);
> 131:             if (!isImplicit() && handle.scope() != this) {

Is this check correct, since an implicit scope could accept a handle from a non-implicit scope?

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PR: https://git.openjdk.java.net/panama-foreign/pull/511